Radio Frequency Identification (RFID), or automatic recognition by contactless data communication, has been used for individual management of goods and articles or entrance/exit management.
According to the automatic recognition by contactless data communication, an IC tag with a unique ID written thereon is attached to an identifying object (an article or a person). Then, a data reader placed at the gate or on the door side supplies electricity to the IC tag by radio waves, and also transmits commands to read the unique ID.
With the IC tag, a silicon chip built therein is driven by the radio waves from the data reader. In response to the commands from the data reader, the IC tag responds by providing information stored in the IC tag, such as the unique ID, with presence or absence of reflection of unmodulated radio waves emitted from the data reader.
The data reader then identifies the presence of the reflected waves as the response from the IC tag, and reads the information stored in the IC tag such as the unique ID.
Now, two or more data readers may be placed in the same neighborhood. In this case, carrier sensing function is used to avoid radio interference among data readers. In order to prevent interference of radio waves emitted from other data readers around, one data reader uses the carrier sensing function with which the one data reader emits radio waves after confirming that no other data reader is emitting its radio waves in the frequency (channel) the one data reader intends to use for emitting radio waves, or that radio interference at the intended frequency (channel) to be used for radio emission is on or below a predetermined threshold.
Therefore, each data reader waits for a certain period of time prior to the start of a data communication with an IC tag, to confirm that no other data reader is emitting radio waves in the intended frequency (channel) to be used for radio emission, or that radio interference in the intended frequency (channel) to be used for radio emission is on or below a predetermined threshold.
Each data reader also waits for a certain period of time, when another data reader is emitting radio waves in the intended frequency (channel) to be used for radio emission, to confirm that the other data reader is not emitting radio waves (the radio emission is finished), or that radio interference in the intended frequency is on or below a predetermined threshold.    Patent Document 1: JP 2000-242742 A